5.2.4Additional lessons
Factors Affecting Rates of Reaction — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 99 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Collision Theory
The rate of reaction depends on how many successful collisions happen in each unit of time.
Collisions needed
- Collisions needed Particles must collide before they can react, so anything that makes collisions more frequent tends to raise the rate.
Enough energy
- Enough energy Only collisions with energy equal to or greater than the activation energy can break bonds and lead to products.
Correct orientation
- Correct orientation Colliding particles must also meet in a suitable geometry, otherwise they simply bounce apart without reacting.
Successful collision
- Successful collision A collision that has both sufficient energy and proper orientation is called a successful or effective collision.
Section 2 of 4Changing Collision Frequency
Changing Collision Frequency
Concentration
- Concentration A higher concentration of a solution packs more particles into the same volume, so collisions happen more often and the rate rises.
Pressure
- Pressure Raising the pressure of a gas squeezes the molecules closer, which increases their concentration and therefore the collision frequency.
Surface area
- Surface area Breaking a solid into smaller pieces exposes more particles to the other reactant, so more collisions occur per second.
Mixing
- Mixing Stirring or shaking brings fresh reactant particles to the reacting surface, which keeps the collision frequency high.
| Factor | Change | Effect on rate | Reason |
|---|---|---|---|
| Concentration | Increase | Increases | More frequent collisions |
| Gas pressure | Increase | Increases | More frequent collisions |
| Surface area | Increase | Increases | More frequent collisions |
| Temperature | Increase | Increases | More frequent and more energetic collisions |
| Catalyst | Add | Increases | Lower activation energy |
Section 3 of 4Temperature and Catalysts
1
Raising the temperature increases the average kinetic energy of the particles, so they move faster and collide more frequently.
2
The more important effect is that a much larger fraction of particles now has energy equal to or above the activation energy.
3
Because of this larger fraction, the number of successful collisions per second rises sharply even for a small temperature increase.
4
A catalyst provides an alternative reaction pathway with a lower activation energy, so more collisions are successful at the same temperature.
5
The catalyst is not used up, and it does not change the position of equilibrium or the overall enthalpy change of the reaction.
Section 4 of 4Exam Checklist & Tips
5.2.4 – Key Points for the Exam
- Rate of reaction The change in concentration of a reactant or product per unit time, for example in mol dm⁻³ s⁻¹.
- Collision theory Reactions occur when particles collide with sufficient energy and the correct orientation.
- Catalyst A substance that speeds up a reaction by providing a lower activation energy pathway and is not consumed.
- Successful collision A collision with energy at least equal to the activation energy and a suitable orientation.
Common Mistakes
- Saying that a higher temperature only works because collisions are more frequent;
- the main reason is the larger fraction of particles above the activation energy.
- Claiming that a catalyst increases the energy of the particles, when it actually lowers the activation energy.
- Stating that a greater surface area changes the activation energy; it only raises the collision frequency.
- Writing that a higher concentration makes the particles move faster, when speed depends on temperature.
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